Various fields of industry including those engaged in the manufacture of electric appliances for domestic use and machines and furniture for office use have recently come to use to a greater extent a weldable black steel sheet having an excellent appearance which can be obtained by the continuous and rapid treatment of a steel sheet plated with zinc or an alloy thereof. A great deal of research and development work has, therefore, been made to obtain such black steel sheets, and includes efforts made to obtain an effective method for the blackening treatment of a steel sheet, and an effective solution which can be used for such treatment.
The following is a summary of the methods which have been proposed for forming a black film on a steel sheet as a result of the past work:
(a) A solution of a resin containing a black pigment, such as carbon black, is applied onto a steel sheet by e.g. spraying or roll coating to form thereon a film having a thickness of several tens of microns.
(b) A film existing on a steel sheet as a result of plating is caused to undergo reaction or electrolysis to develop a black film. This method includes a number of modes as will hereunder be set forth:
(1) A chromating solution containing Ag ions is used to form a black chromate film (as proposed in Japanese Patent Application Laid-Open No. 193376/1983); PA1 (2) A black resin film composed mainly of carbon black is electrolyzed (Japanese Patent Application Laid-Open No. 62996/1981); PA1 (3) A steel sheet which has been electroplated with a Zn-Co, Zn-Ni or Zn-Mo alloy is subjected to anodic treatment (Japanese Patent Publication No. 38276/1986); PA1 (4) A steel sheet which has been plated with a Zn-Ni alloy is subjected to dipping, spraying or anodic treatment with a solution containing nitric acid or a nitrate group to develop a black surface (Japanese Patent Publication No. 30262/1987); PA1 (5) A black plated film is formed by cathodic treatment (e.g. Japanese Patent Application Laid-Open No. 263995/1987); and PA1 (6) Immersion plating is carried out on a surface plated with zinc or an alloy thereof to deposit thereon a metal having a nobler potential (e.g. Japanese Patent Application Laid-Open No. 89879/1987). PA1 (2) The black film which is formed by this method is unsatisfactory in formability. Insofar as carbon black is a conductive pigment, the film is electrically conductive and is, therefore, low in corrosion resistance. Moreover, the film is unsatisfactory in blackness. PA1 (3) and (4) These methods are both uneconomical, since they rely upon the dissolution of metal from the plated layer on the steel sheet. Moreover, the metal ions as dissolved from the plated layer deteriorate the solution for the blackening treatment and can present a serious problem to the continuous treatment. Furthermore, both of the methods have only a limited scope of application, i.e. (3) is applicable only to a steel sheet plated with a Zn-Co, Zn-Ni or Zn-Mo alloy, while (4) is applicable only to a sheet plated with a Zn-Ni alloy. PA1 (5) The black film which is formed by this method is unsatisfactory in formability. PA1 (6) The black film which is formed by this method is unsatisfactory in adhesive strength, and is also low in corrosion resistance, insofar as a metal having a noble or high potential is deposited on a metal having a base or low potential (i.e. on zinc or an alloy thereof).
(c) A treating solution obtained by adding an organic dye to an aqueous solution of potassium sodium silicate is applied onto a surface plated with zinc or an alloy (Japanese Patent Publication No. 30593/1980).
All of these methods do, however, have their own drawbacks, as will hereunder be pointed out.
(a) This is a common method of coating used to form an outer surface coating, the thickness of which is usually at least 10 microns when it consists of a single layer. This thickness is too large to allow the welding of the steel sheet. The thickness of the film which allows the welding of the steel sheet therethrough is from 0.2 to 3 microns. It is difficult to form a satisfactorily black film having a thickness not exceeding three microns, even if the solution may contain the black pigment, such as carbon black, at the maximum possible concentration. In other words, the use of any such blackening agent has been found unable to form any film that is satisfactorily black and yet keeps weldable the steel sheet on which the film has been formed.
(b)(1) The solution containing Ag ions is expensive. Moreover, this method spends as long a time as several tens of seconds in forming a black film and is, therefore, inapplicable to the continuous treatment of a strip which need be finished as rapidly as within five seconds.
(c) This method is not intended for forming a film having an excellently black surface, nor is it intended for imparting weldability, insofar as the disclosure does not contain any specific reference to the film thickness. Moreover, as the film is basically composed of potassium sodium silicate, the hardened film is unsatisfactory in lubricating property as required during its press forming, and is, therefore, unsuitable for any steel sheet that is used for making electrical appliances for domestic use, office machines or furniture, etc. Moreover, the disclosure does not specifically define the organic dye to be used. There are a very large number of organic dyes which are widely different from one another in various properties including light fastness, tinting strength and solubility in a solvent. Some dyes are superior to others in e.g. tinting strength, but inferior in light fastness. It is, therefore, needless to say that all of the organic dyes are equally useful.